LEDs are conventionally known as signaling devices. With the development of high-power LEDs, LEDs are nowadays also used for illumination applications. In such applications, it is important that the LED current is accurately kept at a certain target value, since the light output (intensity of the light) is proportional to the current. This applies especially in so-called multi-color applications, where a plurality of LEDs of different colors are used to generate a variable mixed color that depends on the respective intensities of the respective LEDs: a variation in the light intensity of one LED may result in an unwanted variation of the resulting mixed color.
Driver circuits for driving an arrangement of LEDs with substantially constant current are already known. Typically, such constant current driver circuit comprises a current sensor for sensing the LED current, and a sensor signal is fed back to a controller, which controls a power source such that the sensed current is substantially constant kept at a predetermined level.
Although such control system would normally function satisfactorily, a problem occurs in that the voltage developed over the LED may vary, and that as a result the power source may give an incorrect current. This problem occurs especially in case the power source is a switched mode power source.
The present invention aims to provide a drive circuit where this problem is overcome or at least reduced. More particularly, the present invention aims to provide a drive circuit which is less sensitive to variations in the forward voltage of the LEDs.